1. Field of the Invention
The invention relates to a measuring circuit for the ionic current measurement in the combustion chamber of an Otto engine. The current comprises an ignition transformer the secondary coil of which supplies an ignition circuit which, on the side carrying the positive voltage of the ignition pulse, is connected to ground. On the side carrying the negative voltage of the ignition pulse, it is connected to the ignition spark gaps via a distributor. The invention further comprises, inserted into the ignition circuit, a parallel circuit of a high-voltage Zener diode connected in the non-conducting direction related to the ignition current and of a capacitor as the measuring voltage source.
2. Description of the Prior Art
Irregular variations of the combustion, particularly pressure shocks during the combustion in the combustion chambers of an Otto engine are known as knocking. This is extremely detrimental for the engine and should be avoided.
It is known that pressure variations occuring during knocking become apparent in variations of the ionization and thus of the ionic current across the ignition spark gap. Consequently, it is possible to detect knocking by the measurement of the ionic current within the combustion chamber during or particularly subsequent to the ignition spark.
A circuit of this kind is described in U.S. Pat. No. 4,359,893. Therein the high-voltage Zener diode is connected to the side carrying the negative high-voltage of the ignition pulse of the secondary coil so that the measuring voltage applied to the ignition electrode is positive. The positive measuring voltage on the voltage-carrying electrode of the ignition spark gap is desired per se because one obtains a higher usable signal in the case of usual spark plugs on the basis of their geometrical configuration. However, in the case of this circuit, one needs a special line-path in order to transfer the measuring voltage from the capacitor to the ignition spark gap. Within this line-path, in addition, high-ohmic resistors are required which block the ignition voltage. However, these high-ohmic resistors also reduce the usable measuring signal of the ionic current so that a usable signal is scarcely available. This low measuring signal is to be passed to the analyzing circuits via shielded lines and charge amplifiers. In fact it is possible to use such as arragement in a measuring laboratory, but same is not suitable for standard applications.